Head positioner servo systems for disk files are well understood. For example, the principles of disk file servo systems, as applied to a specific IBM disk file design: the 3340, are explained and compared in an article by Dr. Richard K. Oswald entitled "Design of a Disk File Head-Positioning Servo", IBM Journal of Research and Development, Nov. 1974, pp. 506-512. Disk file positioner servos provide essentially two functions: track seeking or accessing, and track following. Ideally, a track seeking servo configuration should be able to move the data head between two tracks in the minimum possible time, while the track following servo configuration should maintain the head at the centerline of a track being followed. These two quite different functions require different control circuit configurations for practical implementation, even though the same actuator structure, actuator driver circuit and mechanical system components are used for both tasks.
A third, transitional mode, known as track settle, covers the situation where the track seeking servo has completed its task, and the servo circuitry has switched to the track following configuration, but the head transducer has yet to stabilize at the destination track location. Since read and write operations cannot be accomplished until the head has stabilized at the desired track location, it is most desirable to minimize settle time.
Typically, current mode driver circuits are used to drive voice coil actuator structures. The use of a current mode driver circuit results in very fast current and force risetimes within the actuator, leading to improved track access time characteristics. Using a current mode driver, however, requires loop compensation, typically in the form of a lead-lag phase compensation network. The lead-lag network provides necessary damping and prevents the servo loop from appearing to be a simple harmonic oscillator.
In rotary voice coil actuator structures, such as shown in commonly assigned U.S. patent application Ser. No. 07/192,353, now U.S. Pat. No. 5,005,089, filed on May 10, 1988, bias forces are present which bias the actuator in one direction. One source of bias force is presented by a flexible circuit structure which is provided to connect the actuator coil and head read/write preamplifier and switching circuitry to stationary circuitry mounted to the base. These bias forces may be considered as an offset signal which adds to or subtracts from the actuator current used to correct head position to track centerline during track following operations.
With non-zero bias force, the lead-lag compensator network of the track following servo loop must integrate the position error in order to provide a proper drive current. This integration classically creates a long settle time or "tail" in the settling position error signal because the integrator circuit gets charged through a very large series resistor from a source of the PES. Reducing the value of the resistor during track following operations reduces the phase margin and may result in instability of the track following servo loop during track following.
Thus, a hitherto unsolved need has arisen for a disk file head position servo loop which may dynamically change loop compensation characteristics during track settling and which maintains a sufficiently wide phase margin during track following to provide stability to the actuator structure.